Natural gas as produced from a gas well presents a separations challenge. Often the natural gas is found together with other components such as sulfur compounds, water, and associated gases. The associated gases found in natural gas typically include carbon dioxide, nitrogen, helium, argon, and the like. Generally, these other gases are separated from the natural gas by bulk methods employing membrane systems. Permeable membrane processes and systems are known in the art and have been employed or considered for a wide variety of gas and liquid separations. In such operations, a feed stream is brought into contact with the surface of a membrane, and the more readily permeable component of the feed stream is recovered as a permeate stream, with the less readily permeable component being withdrawn from the membrane system as a non-permeate stream.
The inherent simplicity of such fluid separation operations constitutes an incentive in the art to expand the use of membrane systems in practical commercial operations. In this regard, it will be appreciated that the selectivity and permeability characteristics of such membrane systems must be compatible with the overall production requirements of a given application. It is also necessary, of course, that the membranes exhibit acceptable stability and do not suffer undue degradation of their performance properties in the course of practical commercial operations.
For example, in air separation applications which constitute a highly desirable field of use for permeable membranes, oxygen is typically the more readily permeable component of the feed air for particular membranes and is withdrawn as the permeate gas. In such embodiments, nitrogen is the less readily permeable component and is recovered as non-permeate gas. In air separation applications, it has been found that the performance characteristics of the membranes are sensitive to the presence of certain contaminants in the feed air stream. Exposure to such contaminants may result in a significant reduction in the permeability of the membrane in use. Fortunately, most contaminants commonly present in ambient air, such as light hydrocarbons, H.sub.2 O, and CO.sub.2, have been found to result in, at most, a modest decrease in membrane permeability. The presence of even relatively low concentrations, e.g., less than 1 ppm by volume as C.sub.1, of heavy hydrocarbon oil vapors, such as might enter the feed air stream from an oil lubricated air compressor, can result in rapid and extensive loss of membrane permeability.
It is well known in the art that selection of oil lubricated rotary screw feed compressors for the membrane permeability is subject to an initially rapid and significant decrease, followed by a further gradual decline over a period of months of operation. In response to such an undesirable decrease in membrane permeability, it is presently common membrane practice to size the active membrane surface area with a safety factor sufficiently large to compensate for the anticipated permeability loss from all sources. Initially, therefore, the membrane system is significantly oversized for the desired product flow, and the feed gas compressor is typically operated in a turndown mode. As permeability degradation proceeds, either the operating temperature or pressure, or both, are increased to compensate for the decrease in permeability. In some instances, it is necessary or desirable to by-pass some of the modules in the membrane system initially so as to reduce excess membrane area employed when the membranes exhibit their full permeability capability and subsequently to bring such by-passed modules on stream as degradation of the initially employed modules progresses. In such instances, it will be appreciated that, in addition to a significant capital cost penalty associated with the provision of extra membrane surface area, such a membrane system must operate over a significant portion of its operating life under off design conditions and that the control strategy for such a membrane system is more complex than for a system operating closer to its optimum design conditions.
As an alternative to such overdesign of membrane systems to compensate for degradation in use, attempts have been made to restore lost performance, but such efforts were initially unsuccessful in developing an economically feasible means for restoring the permeability of degraded membranes. Restoring any portion of the degraded membranes would require interruption of the gas treating operation, displacing large quantities of gas. Neither overdesign of the membrane system nor interruption of gas product operations for membrane restoration treatment, or a combination of these approaches is an entirely satisfactory means for overcoming permeability degradation in practical commercial air or other gas separation operations. Further improvement in the response to the problem of membrane degradation is highly desirable in the membrane art.
U.S. Pat. No. 4,881,953 to Prasad et al. discloses another approach to the problem of preventing premature loss of membrane capacity by passing the feed gas mixture through a bed of adsorbent material, such as activated carbon to adsorb contaminants such as heavier hydrocarbon contaminants without the removal of lighter hydrocarbons. Prasad requires that a means for removing moisture from the feed gas be provided because high moisture levels generally limit the ability of activated carbon adsorbents to retain their adsorptive capacity for heavy hydrocarbons.
Generally, thermal swing processes utilize the process steps of adsorption at a low temperature, regeneration at an elevated temperature with a hot purge gas and subsequent cooling down to the adsorption temperature. One process for drying gases generally exemplary of thermal swing processes is described in U.S. Pat. No. 4,484,933, issued to Cohen. The patent describes basic thermal swing processing steps coupled with the use of an auxiliary adsorber bed for improving the regeneration step. Thermal swing processes are often used for drying gases and liquids and for purification where trace impurities are to be removed. Often, thermal swing processes are employed when the components to be adsorbed are strongly adsorbed on the adsorbent, i.e., water, and thus, heat is required for regeneration.
It is an object of the invention, therefore, to provide an improved membrane system and process for overcoming the problem of degradation of permeability during gas production operations.
It is another object of the invention to provide a membrane system and process obviating the need for significant overdesign or for premature replacement of degraded membrane modules.
It is a further object of the invention to provide a membrane system and process for maintaining membrane permeability and minimizing the need for the interruption of gas producing operations for the treatment of membrane modules for restoration of the permeability characteristics thereof.